In the art of machining gears and fluted rotors for pumps, compressors and the like it is important that the shape or profile of the gear teeth or rotor lobes be formed with great accuracy. Such accuracy of the profile shape is important for proper interaction or meshing of the teeth or lobes and for achieving suitable operating efficiency for fluid handling devices such as fluted rotor pumps and compressors. The machining of tooth or flute profiles by precision form milling has been a known and accepted method for the manufacture of fluted rotors for helical screw compressors and pumps as well as gears, splines, sprockets and other toothed machine elements. In conventional form milling operations one flute or tooth space is cut completely and the workpiece is then rotatably indexed with respect to the cutter to the next flute position and the cutting operation repeated. In form milling of helical screw compressor rotors, for example, the milling cutter is mounted for rotation about an axis which is substantially perpendicular to the helix line of the rotor flutes. The workpiece or rotor is advanced with respect to the cutter in a direction parallel to the rotor longitudinal axis and is rotated in timed relation to the axial feed to form the helical flute. When one flute is completely formed the workpiece is returned to the starting point of the operation and rotatably indexed to a position for cutting another flute. In the manufacture of helical screw rotors the form milling process provides suitable profile accuracy and can be used for unsymmetrical as well as symmetrical rotor profiles. Form milling processes are, however, relatively slow and the periodic indexing of the workpiece from one flute to the next introduces a greater chance for error in flute or tooth spacing.
The process known as hobbing may be used for symmetrical and unsymmetrical flute or tooth profiles but must be associated with workpieces which have uniform spacing of the flutes or teeth. Hobbing, which is widely used for cutting gears, may obtain acceptable profile accuracy at relatively high rates of production when compared to form milling. Moreover, since the workpiece is continually indexed during one pass of the cutter, accurate spacing of the flutes or teeth is more easily obtained and the workpiece is finished in one continuous operation.
However, with integrally formed hobs wear and the ever present danger of damaging a cutting blade causes great expense in tooling reconditioning and replacement, particularly for hobs used to accurately cut relatively large gears and rotors for helical screw machines.
It has been suggested that hobs may be made with replaceable cutting blade elements. Such arrangements are discussed in U.S. Pat. No. 2,498,721 to S. M. Stafford and U.S. Pat. No. 3,740,808 to K. Shioya. The hobs disclosed in the aforementioned patents require that the cutting blades be ground to their finished form before being inserted in place on the cutter body. Grinding the blade elements to the desired profile form or sharpening the edges while the blade elements are in place on the body is extremely difficult due to the plural rows of blades wherein axially adjacent blades interfere with grinding a particular cutting edge on a blade element. Furthermore, the problem of precisely locating the finish ground blade elements on the cutter body very often results in less than acceptable profile accuracy and uneven wear on the blade elements. Heretofore, the benefits derived from high production rates using the hobbing process have been substantially negated by the high initial cost of hobbing cutters as well as the costs associated with maintenance and replacement of the cutters. This has been found to be particularly true when considering the hobbing process for the manufacture of rotors for helical screw fluid handling machines, namely pumps, compressors, and expanders. The condition applies just as well, in varying degrees, to the manufacture of gears, splined shafts, chain sprockets, and various other machine elements.